Camera traps placed to detect predators underestimate prey densities

Mountain ungulates play an important role in ecosystems as primary consumers and as prey for rare predators. Monitoring their populations is therefore critical for conservation efforts. Within the 12 countries comprising the range of the snow leopard Panthera uncia, camera traps are routinely deployed to estimate numbers of this apex predator, providing an opportunity to also estimate numbers of their prey using bycatch data. However, the relative accuracy of the resulting prey density estimates compared to field surveys targeted specifically at prey species was unknown. We compared the performance of distance sampling based on camera-trap data with field surveys to estimate population densities of bharal Pseudois nayaur. We assessed estimates of bharal numbers from cameras placed to detect snow leopards (where ungulate captures presented bycatch data) against estimates from cameras placed specifically to detect bharal and then compared both with an independent estimate of bharal density from double-observer surveys and a total count of all bharal in the study area. The double-observer field surveys suggested a density of 1.94 bharal/km², which was similar to the density derived from the total count (1.92 bharal/km²). By comparison, we estimated density to be 2.11 bharal/km² from camera-based distance sampling and 0.35 bharal/km² from cameras placed to detect snow leopards (bycatch data). The density estimate from the ungulate bycatch data was significantly lower than that from the double-observer field survey and from the total count. It was also less precise, more costly and more time-consuming to obtain. Our results caution against using bycatch data from surveys designed for predators to estimate ungulate prey densities and indicate that tailored survey methods are required.